Looks like it's the mobile-specific tweak from T311836 to make the reply/cancel block line up with the advanced-toggle, rather than being inline with the footer-text.

It was working shortly after that patch was merged in https://patchdemo.wmflabs.org/wikis/d0c3125da5/w/index.php?title=Talk:DiscussionTools&mobileaction=toggle_view_mobile

This turns out to actually be fallout from the enter-warning for T326500 -- the CSS expects that the advanced block is immediately followed by the actions, and the enter-warning element now exists between them. I'll tweak that.

It's this rule:

.skin-minerva .ext-discussiontools-ui-replyWidget-advanced:not(.oo-ui-element-hidden) + .ext-discussiontools-ui-replyWidget-actionsWrapper .ext-discussiontools-ui-replyWidget-actions {

Change 963338 had a related patch set uploaded (by DLynch; author: DLynch):

[mediawiki/extensions/DiscussionTools@master] Mobile reply buttons were overlapping the expanded advanced options

https://gerrit.wikimedia.org/r/963338

Change 963338 merged by jenkins-bot:

[mediawiki/extensions/DiscussionTools@master] Mobile reply buttons were overlapping the expanded advanced options

https://gerrit.wikimedia.org/r/963338

✅ Reply button does not overlap "Advanced" comment summary box on mobile